Using SET TRANSACTION
You use the SET TRANSACTION statement to begin the read-only or read-write transaction, start an isolation level, or assign your present transaction to a specified rollback segment. The Read-only transactions are helpful for running the multiple queries against one or more tables whereas other users update the same tables.
During the read-only transaction, all the queries refer to similar snapshot of the database, providing the multi-query, multi-table, read-consistent view. The Other users can carry on to query or update data as usual. The commit or rollback ends the transaction. In the illustration below, like the store manager, you use the read-only transaction to meet the sales figures for the day, the past week & the past month.
The figures are unaffected by the other users updating the database throughout the transaction.
DECLARE
daily_sales REAL;
weekly_sales REAL;
monthly_sales REAL;
BEGIN
...
COMMIT; -- ends previous transaction
SET TRANSACTION READ ONLY;
SELECT SUM(amt) INTO daily_sales FROM sales
WHERE dte = SYSDATE;
SELECT SUM(amt) INTO weekly_sales FROM sales
WHERE dte > SYSDATE - 7;
SELECT SUM(amt) INTO monthly_sales FROM sales
WHERE dte > SYSDATE - 30;
COMMIT; -- ends read-only transaction
...
END;
The SET TRANSACTION statement should be the first SQL statement in the read-only transaction and can only come out once in the transaction. When you set a transaction to the READ ONLY, the following queries see only changes committed before the transaction began. The use of READ ONLY does not affect the other users or transactions.